CONTACT:Bob Kuska
National Institute of Dental and Craniofacial Research

Nature’s Secrets Yield New Adhesive Material

Scientists report they have merged two of nature’s most elegant
strategies for wet and dry adhesion to produce a synthetic material
that one day could lead to more durable and longer-lasting bandages,
patches, and surgical materials. As published in this week’s issue
of the journal Nature, the scientists, supported by the
National Institute of Dental and Craniofacial Research (NIDCR),
part of the National Institutes of Health, have designed a synthetic
material that starts with the dry adhesive properties of the gecko
lizard and supplements it with the underwater adhesive properties
of a mussel. The hybrid material, which they call a geckel nanoadhesive,
proved in initial testing to be adherent under dry and wet conditions.
It also adhered much longer under both extremes than previous gecko-based
synthetic adhesives, a major issue in this area of research.

According to the authors, their findings mark the first time that
two polar opposite adhesion strategies in nature have been merged
into a man-made reversible adhesive. “Our work represents a proof
of principle that it can be done,” said Phillip Messersmith, D.
D.S., Ph.D., a scientist at Northwestern University in Evanston,
Ill. and the senior author on the paper. “A great deal of research
still must be done to refine the fabrication process and greatly
reduce its cost. There’s no reason to believe that these improvements
can’t be achieved, but it’s going to take time.”

Dr. Messersmith said the inspiration for the geckel nanoadhesive
came about two years ago when he noticed an article about the adhesive
force of a single hair from the foot of gecko. As lizard fans have
long marveled, geckos climb walls and other dry, steep surfaces
not by producing a glue-like substance but through a natural adaptation
of the hairs of that cover the soles of their feet.

Roughly one-tenth the thickness of a human hair, each gecko hair
splits multiple times at the end. These split ends contain cup-like
structures called spatulae that vastly increase the hair’s surface
area. Whereas a human hair touches a surface just once, the gecko
makes multiple contacts with the suction-like spatulae. With roughly
a half million hairs on each foot, scientists estimate a gecko
has a billion spatulae at work as it scampers up a wall.

Messersmith knew that researchers have attempted for several years
to produce synthetic adhesives based on the adherence strategy
of the gecko. What caught his eye in this article is gecko adhesion
doesn’t work well in water. Messersmith, who studies the underwater
adhesion of mussels, had an idea. What if each synthetic gecko-inspired
polymer, called a pillar, was coated with a man-made adhesive protein
inspired by the mussel? As Messersmith mused, nobody had ever tried
it and, if successful, this hybrid approach might spawn a new and
potentially superior direction in designing temporary adhesive
materials.

As reported in Nature, Messersmith’s idea turned out
to be correct. He and his colleagues designed a small nanopolymer
array that mimicked the natural spatial patterns of the hair on
the foot of a gecko. They then coated their creation with a thin
layer of a synthetic compound. This unusual compound mimics the
reversible bonding action of a mussel adhesive protein that Messersmith’s
group has studied for the past several years.

In their initial experiments, which were led by graduate student
Haeshin Lee, they found that the wet adhesive force of each pillar
increased nearly 15 times when coated with the mussel mimetic and
applied to titanium oxide, gold, and other surfaces. The dry adhesive
force of the pillars also improved when coated with the compound.

“That actually wasn’t so surprising to us,” said Lee, the lead
author on the study. “The mussel-inspired adhesive is extremely
versatile in that it can bond reversibly to inorganic surfaces
under wet and dry conditions.”

As Lee noted, the next research hurdle was whether their hybrid
geckel nanoadhesive would continue to stick to surfaces after multiple
contacts. This has been a major challenge with other gecko-based
adhesives. They typically stick well at first but lose their ability
to adhere after a few cycles of contact with a tipless cantilever.

Using the cantilever and repeatedly touching it down, Lee developed
a camera to visualize the process down to individual pillars. He
found that the geckel hybrid maintained 85 percent of its adherence
under wet conditions after 1,100 contacts with the tip. Under dry
conditions, the level of adherence was 98 percent.

“This isn’t quite a home run, but it’s somewhere in between a
double and a triple,” said Lee, who devised on his own a special
imaging devise to visualize individual pillars during the experiments.

Messersmith said that while the results are extremely promising,
his group still must tackle several practical problems before it
can scale up its research. “Any time that you fabricate an array
of nano pillars of this type over large areas, you must have a
very effective way of doing it without losing the efficacy of the
approach,” said Messersmith. “We’ll also need to reduce the fabrication
costs to make geckel commercially viable.”

But Messersmith said he envisions great possibilities for geckel. “Band
aids already adhere well, except if you go swimming, take a shower,
or somehow expose it to a lot of water,” said Messersmith. “So
I think the most important thing with this adhesive is the added
value of resisting immersion in water.”

“I should add that the essential component of the wet adhesive
polymer on the pillars contains a chemical that we have discovered
last year adheres well to mucosal surfaces, such as those inside
our mouth,” he noted. “It may be possible to develop patches in
the future that can be applied on the inside of the cheek to cover
damaged tissue.”

The National Institute of Dental and Craniofacial Research is
the Nation’s leading funder of research on oral, dental, and craniofacial
health.

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